Method for efficient packet framing in a communication network

ABSTRACT

Techniques to reduce the transmission overheads in a communication system are disclosed. In an embodiment, a method described herein relates to the elimination of redundant padding to realize an integer number of FEC code-words during the FEC-encoding process of transmission as well as the reduction/elimination of redundant padding to realize an integer number of transmission symbols during the subcarrier modulation mapping process of transmitting OFDM/ACMT/DMT symbols. The techniques are described in the context of a communication system based on the MoCA specification. Furthermore, techniques for channel-profiling, channel-estimation and bandwidth request/grant signaling that facilitate the realization of the method of reduction of transmission overheads in a MoCA system are also described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is entitled to the benefit of provisional U.S. PatentApplication Ser. No. 61/042,586, filed Apr. 4, 2008, the disclosure ofwhich is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The invention relates to a communication system and more particularly toa communication system using adaptive constellation multi-tone (ACMT)modulation.

BACKGROUND OF THE INVENTION

Driven by the increasing prevalence of digital content and multi-mediaapplications, of late there has been a dramatic growth in the need forhome networking. This has fuelled new development of home networkingtechnology both wired and wireless. One such technology—Multimedia overCoax Alliance (MOCA) V1.0 Specification (identified herein as [1] or‘the standard’) describes the MAC and PHY layers for high-ratecommunications over the coaxial TV-cable plant that is present in mosthomes. MoCA makes use of a 256-tone OFDM based-PHY to provide for adata-rate of up to 310 Mbps at a range of up to 300 feet on a 50 MHzchannel. In order to provide for reliable communications, the standardspecifies the use of a Reed Solomon (RS) forward error correction schemedrawn from Galois Field GF(256) with code-words having sizes chosen fromthe set {(32,40), (36,44), (64,74), (128,140), (192,208)} as specifiedin the standard. The respective byte-error correction capabilities forthese codes are {4, 4, 5, 6, 8} respectively. Thus, the (32,40)code-word can correct 4 byte-errors in a block of 32 information-bytesusing 8 parity-bytes, while the (192,208) code-word can correct for 8byte-errors in a block of 192 information-bytes using 16 parity-bytes.Of the code-words specified in the standard, the (36,44) code is usedonly for beacon transmission and not for data transmission.

FIG. 1 (Prior-art) depicts the steps carried out by a standard complianttransmitter in converting a MAC-packet to a PHY-packet for transmissionover the channel. MAC-frame 101 depicts a packet which is handed to thePHY for transmission. The PHY performs FEC-padding by appendingredundant pad information 106 to the MAC-frame 101 to produce resultantFEC-padded frame 105.

The FEC-padded frame 105 is encrypted to produce the encrypted-frame110. The encrypted-frame 110 is FEC-encoded into individual code-blockseach code-block constituted of a data-section and a parity-section. Asan example, we depict the encoding of encrypted-frame 110 into two FECcode-blocks—116 and 117, each of which is constituted ofdata-section—116 a and 117 a, and parity-section—116 b and 117 b,respectively. The collective FEC-encoded frame is referred to asFEC-encoded frame 115 in FIG. 1.

The FEC-pad 106 applied to MAC-frame 101, above is determined such thatthe eventual FEC-encoded frame can be constituted of an integer numberof FEC code-words.

An ACMT-pad 121 comprising of redundant pad information is appended toFEC-encoded frame 115 to produce an ACMT-padded frame 120, as shown inFIG. 1. The ACMT-padded frame 120 is Byte scrambled to produce aByte-scrambled frame 125, as shown in the Figure.

The Byte-scrambled frame 125 is further decomposed into an integernumber (three as per this example) ACMT symbols—130 a, 130 b and 130 c,collectively called the Subcarrier modulation mapped frame 130.

The ACMT-pad 121 applied to FEC-encoded frame 115, above is determinedsuch that the eventual Subcarrier modulation mapped frame 130 can beconstituted of an integer number of ACMT symbols.

The Subcarrier modulation mapped frame 130 is bin-scrambled to produce abin scrambled frame 135. The PHY performs ACMT modulation on 135 andinserts the appropriate preamble 141 to generate the ACMT Modulatedframe 140. Frame 140 is further filtered and up-converted to theappropriate RF-carrier frequency to generate the final PHY packet 145,which is transmitted over the channel.

A MoCA standard compliant receiver receives the transmitted PHY packet145 and demodulates, decodes and decrypts the packet to recover theoriginally transmitted MAC-frame 101.

The MoCA PHY makes use of an adaptive constellation multi-tone (ACMT)modulation scheme whereby a transmitter modulates each tone of itsOFDM-symbol differently in accordance with the SNR expected for thattone at the receiver, for a particular (transmitter, receiver) pair.Pre-requisite to using ACMT-modulation is the profiling of the channelbetween all-pairs of nodes in the network. The standard defines a meanswhereby a new-node (NN) joining a network performs modulation profilingwith all existing nodes (ENs) in the network, allowing the NN and ENs todetermine the per-tone bit-loading pattern to be used for communicationbetween them. Additionally, nodes (NN and ENs) also determine thepreamble-type to be used for data communication between them.

Nodes refresh their profile information during periodic link maintenanceoperations (LMOs) as specified in the standard. In addition to updatingtheir modulation profiles and preamble-types, MoCA nodes also determinethe delay-spread of the channel between them and their peer-nodes andcorrespondingly adjust their cyclic-prefix in order to compensate forthe same.

The modulation profiling of the channel between two nodes is performedby the transmitter sounding the channel with a packet comprising256-tone ACMT symbols, referred to as a ‘Type-1 Probe’ in the standard.The receiver determines the per-tone SNR on each tone and determines itsbit-loading capacity. It also determines the preamble-type to be usedfor subsequent transmissions from the transmitter. The determinedper-tone bit-loading capacity and preamble-type are fed-back to thetransmitter by means of a ‘Type-1 Probe Report’ as described in thestandard. The transmitter uses this modulation profile to effectsubsequent transmissions. The sum of the per-tone bit-loading capacitiesacross all tones in the 256-tone ACMT symbol is equivalent to the numberof bits per ACMT symbol defined as Nbas in the standard and henceforthreferred to as Nbas256.

Likewise, the transmission of Type-3 Probes and Type-3 Probe reports asdefined in the standard are used to determine the cyclic prefix to beused.

Collectively, the modulation-profile, preamble-type and cyclic-prefix tobe used for a transmission/reception are referred to as a ‘PHY-Profile’.

Depending on the nature of the transmission and its recipients, thestandard specifies the use of different PHY-profiles between two nodes.In general, the specific bit-loading pattern, preamble-type andcyclic-prefix to be used for communication between two nodes may beidentified by the 3-tuple comprising: the source node identifier, thedestination node identifier and the PHY-profile identifier.

Nodes in a MoCA network exchange data with one another using aTDMA-based MAC protocol. One of the nodes in the network is designatedas the network coordinator (NC)—which in addition to transacting data onthe network, is responsible for coordinating medium-access among allnodes on the network, among other functions defined in [1]; while theother nodes are referred to as existing-nodes (ENs).

An EN, with data to transmit to another node first transmits areservation-request (RR) to the NC. A RR may consist of a plurality ofRequest Elements, each of which reserves bandwidth for a particulartransmission. The standard specifies two types of Requestelements—Asynchronous Data/Control Reservation Request element andLink-Probe Reservation Request element. The Asynchronous Data/ControlReservation Request element is used for reserving bandwidth forupper-layer data and MoCA control frame transmissions, while the LinkProbe Reservation Request Element is used for reserving bandwidth forprobe transmissions.

As per the standard, the Asynchronous Data/Control Reservation RequestElement comprises information elements as listed in the structure below:

Asynchronous Data/Control Reservation Request Element :=    {      FRAME_SUBTYPE       FRAME_TYPE       DESTINATION       PHY_PROFILE      REQUEST_ID       PARAMETERS       PRIORITY       DURATION    }

The NC computes a schedule for transmission based on the ReservationRequest Elements received from nodes in its network during a schedulinginterval referred to as a ‘MAP-cycle’ in the standard. The NC furtherbroadcasts a ‘MAP-frame’ which defines the schedule for all mediumactivity in the subsequent MAP-cycle to all ENs in the network. Nodes inthe network then transmit and/or receive data in accordance with theschedule of the MAP-frame.

A MAP-frame is comprised of a plurality of allocation-units (AUs), eachof which specifies an allocation of time on the medium to a transmissionas requested via a request element. The standard specifies two types ofAUs—Probe Allocation Unit (PAU) and Data Allocation Unit (DAU)respectively.

A PAU is used to allocate time/bandwidth to a probe transmission.

A DAU is used to allocate bandwidth to data and control traffic on thenetwork, providing information about the start-time, the type oftransmission to be scheduled and the profile-identifier, along with thesource and destination node IDs for the transmission. As per thestandard, a DAU comprises of information elements as listed in thestructure below:

Data Allocation Unit :=    {       FRAME_SUB_TYPE       FRAME_TYPE      SRC       DESTINATION       PHY_PROFILE       REQUEST_ID      IFG_TYPE       OFFSET    }

In accordance with the methods of the standard, the process of computingthe FEC-pad 106 at the transmitter is such that the receiver needsknowledge only of the number of bits per ACMT symbol and the number ofACMT symbols in the PHY data packet payload in order to unambiguouslydetermine the number and sizes of the RS code-words in the packet,thereby setting up the receiver for correct reception.

A MoCA receiver may determine the number of ACMT symbols to be receivedbased on the difference in the OFFSET field between successiveallocation-units in the MAP-frame, as per the method in the standard.Likewise, a receiver may determine the number of bits per ACMT symbol,the preamble-type and the length of cyclic-prefix based on thePHY_PROFILE, SRC and DESTINATION fields specified in the DAU.

As per FIG. 1, a MoCA PHY packet comprises of a preamble 141 and the PHYdata payload 142. While payload 142 carries the encrypted, encoded,scrambled and modulated MAC-data, preamble 141 comprises of a knownsequence. The various parts of preamble 141 are used to facilitatevarious aspects of packet acquisition including AGC gain settling,symbol timing estimation, frequency offset estimation etc. The channelestimation sequence (CES) is used by the receiver to derivechannel-estimates, which are subsequently used to equalize the payloadACMT symbols prior to demodulation and decoding. The MoCA preamblesspecified in the standard make use of a CES based on 256-tone ACMTsymbols.

While MoCA [1] was originally designed to provide a usable MAC-layerthroughput of 125 Mbps, it was soon determined that higher throughputswere required to support the evolving ‘bandwidth-hungry’ applications onhome networks. MoCA V1.1 Draft Specification (referred to herein as [2])was defined as a set of MAC-layer extensions to [1] that among otherfunctionality, augmented the MAC-layer throughput of [1] to 180 Mbps.However, this still falls short of requirements set by newer networkusage scenarios, which require even higher PHY data-rates.

SUMMARY

A method for reducing the FEC-pad overhead required to encode adata-frame to form an FEC-encoded frame, the reduction of said FEC-padresulting in a shortened FEC encoded frame, said method comprising thestep of determining a number of FEC code-words that minimizes theparity-overhead of the FEC-encoded-frame, the step of determining anFEC-pad of known values to be appended to the data-frame, said FEC-padresulting in the shortest possible last FEC code-word in resultantFEC-encoded frame, the step of determining a shortened last FECcode-word from said last FEC code-word, said shortened last FECcode-word comprising the data of last FEC code-word without FEC-pad; andthe parity of last FEC-code-word, the step of determining a shortenedFEC-encoded frame, said shortened FEC-encoded frame comprising of afirst group of code-words corresponding to all but the last FECcode-word of the FEC-encoded frame and a shortened last FEC code-word.Additionally, the method of the decoding said encoded frame, said methodcomprising the step of inserting an FEC-pad of known values in betweenthe data and parity of the shortened last FEC-code-word of the shortenedFEC-encoded frame, resulting in an FEC-encoded frame, the step ofdecoding said FEC-encoded-frame to determine an FEC-padded data-frame,and the step of discarding said FEC-pad to recover the under-lyingdata-frame. Furthermore, the method can further comprise the step ofdetermining a number of ACMT-symbols that reduces the medium-occupancyof the associated ACMT modulated frame, and the step of determining areduced ACMT-pad to be appended to the data-frame, the resultantACMT-padded data-frame when modulated resulting in the number ofACMT-symbols determined above, the resultant ACMT symbols being of sizesthat reduce medium occupancy, the aggregate of the ACMT modulatedsymbols determined above referred to as a shortened ACMT modulatedframe. In addition, the method can further comprise the step ofdetermining a reduced ACMT-pad to be appended to the data-frame theresultant ACMT-padded data-frame when modulated resulting in the numberof ACMT-symbols determined above, all-but-last of the resultant ACMTsymbols being of size corresponding to the largest ACMT symbol, the lastACMT symbol being of size less than or equal to the largest ACMT symbol,the aggregate of the ACMT modulated symbols determined above referred toas a shortened ACMT modulated frame.

Another embodiment is a method for generating a PHY-packet from adata-frame, prior to transmission, said method comprising the step ofdetermining an FEC-pad of known values that results in a minimum parityoverhead when encoding the data-frame above to form an FEC-padded frame,the step of FEC-encoding said FEC-padded frame to determine an FECencoded frame, the step of determining a shortened FEC encoded frame byshortening said FEC encoded frame, the step of determining an ACMT-padthat when appended to said shortened FEC encoded frame, results in anACMT-padded frame, the step of determining a shortened ACMT-modulatedframe by modulating said ACMT-padded frame to the minimum number ofACMT-symbols, having the minimum medium occupancy, and the step ofdetermining a PHY-packet from said ACMT-modulated frame. Furthermore themethod can further comprise the step of determining an encryption-padthat when appended to the MAC-frame results in an encryption-paddedframe, the step of encrypting said encryption-padded frame to determinean encrypted frame, and the step of determining an FEC-padded-frame, aFEC encoded frame, a shortened FEC encoded frame, an ACMT pad, anACMT-padded frame, a shortened ACMT modulated frame.

Additionally, a system comprising a transmitter and a receiver isdisclosed wherein the transmitter and the receiver are configured todetermine the modulation profile for a reduced size ACMT symbol by thetransmitter transmitting a legacy modulation profiling sequence to thereceiver, and the receiver using received modulation profiling sequenceto determine the signal to noise ratio of the legacy tone-positions,determining the signal to noise ratio at the tone positions of thereduced size ACMT symbol by interpolation of the signal to noise ratiosof the legacy tone-positions, and communicating the determinedmodulation profile of the reduced-size ACMT symbol to the transmitter.

Also disclosed is a system comprising a transmitter and receiverconfigured to determine the modulation profile for a reduced size ACMTsymbol from the modulation profile of a legacy ACMT symbol by thetransmitter and receiver applying a common and pre-determined set ofrules to the modulation profile of the legacy ACMT symbol to determinethe modulation profile of the reduced-size ACMT symbol. Furthermore, thetransmitter and receiver can apply a common set of rules where thebit-loading capacity of a tone of a reduced size ACMT symbol isdetermined as the minimum of the bit-loading capacities of the adjacenttones of the legacy ACMT symbol, as determined from its modulationprofile.

Additionally, a transmitter can be configured to determining theparameters required by the receiver to receive a data-packet transmittedusing the method for generating a PHY-packet described above. Thetransmitter communicates appropriate parameters prior to transmission ofsaid data-packet, where the parameters comprises the preamble-type inuse by the transmitter, the modulation-profile used by the transmitterin transmitting the packet, the cyclic-prefix in use by the transmitterfor every ACMT symbol, the duration of the transmission, and theACMT-pad used by the transmitter in its transmission. The transmitterfurther can be configured to transmit information to the receiver via athird-node in the network. The transmitter further can determine theparameters by selecting the preamble-type, modulation-profile andcyclic-prefix to be used based on the source, destination and type ofthe packet to be transmitted, determining the size of an encryption-padto be applied to the packet so that the resultant encryption-paddedpacket may be encrypted to form an encrypted packet, determining thesize of a FEC-pad to be applied to the encrypted-packet so that theresultant FEC-padded packet may be encoded to obtain a shortenedFEC-encoded frame that has a reduced parity overhead, and determiningthe size of an ACMT-pad to be applied to the shortened FEC-encoded framesuch that the resulting ACMT-padded frame may be modulated using theavailable symbol-sizes to construct a shortened ACMT modulated framethat when transmitted minimizes the medium occupancy, the resultantmedium occupancy determined to be the duration.

The receiver corresponding can be configured to determine the parametersof a packet to be received based on the information received from thetransmitter by determining the number of ACMT symbols to be receivedbased on the duration of the packet, the preamble type and thecyclic-prefix, determining the size of the various ACMT symbols usingthe determined duration and the determined number of ACMT symbols,applying the knowledge of the methods of the transmitter in determiningthe ACMT modulated frame, determining the length of the shortened FECencoded frame using knowledge of the modulation-profiles of the varioussymbol-sizes along with the sizes of the ACMT-symbols, and determiningthe size of the various FEC code-words and the FEC-pad to be inserted inthe shortened FEC code word so as to recover the FEC encoded-frame.

The receiver can also be configured to determine the channel-estimatesat the tone-positions of a shortened ACMT-symbol based on channelestimates at the tone positions of legacy ACMT symbols as determinedfrom the legacy preamble by determining the size of the shortened ACMTsymbol prior to receiving it and interpolating the channel estimates atthe tone positions of the legacy ACMT symbol to determine the channelestimates at the tone positions of the shortened ACMT symbol.

Other systems, methods, features, and advantages of the presentdisclosure will be or become apparent to one with skill in the art uponexamination of the following drawings and detailed description. It isintended that all such additional systems, methods, features, andadvantages be included within this description, be within the scope ofthe present disclosure, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF DRAWINGS

Many aspects of the disclosure can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present disclosure. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 depicts a convention method of framing a packet in a MoCAnetwork;

FIG. 2 depicts the method of code-shortening;

FIG. 3 depicts the method of transmitting a shortened tail-symbol;

FIG. 4 depicts the method of transmitting a packet across a MoCAnetwork;

FIG. 5 depicts the method of determining thebit-loading/channel-estimates of shortened ACMT symbols;

FIG. 6 is a flowchart depicting the method of a transmitter to generatea reservation request element; and

FIG. 7 is a flowchart depicting the method of a receiver to determinethe receiver parameters.

DETAILED DESCRIPTION

Nodes are configured to realize a more optimal packet framing structure,resulting in a reduction (and in some instances, elimination) ofredundant pad information in the MoCA PHY packet. This leads to anoverall reduction in medium occupancy, the resulting savings beingavailable for other transmissions, thereby resulting in an overallincrease in throughput of the network.

Furthermore, these nodes are designed to coexist and interoperate withlegacy nodes in the network. It is understood that legacy nodes arenodes in the network which have not been configured to realize a moreoptimal packet framing structure as described in this disclosure.

Each of the code-words specified by the standard has a different errorcorrection capability in terms of number of byte-errors that can becorrected in a code-word. As a PHY packet can be constituted of a numberof RS code-words above, the code-word with the lowest error-correctioncapability per unit-information is sufficiently robust to meet thetransmission reliability needs of the over-lying MAC andapplication-layers. The number of code-blocks to be used is chosen in amanner that minimizes the overall number of code-blocks and the amountof parity-information to be associated with a payload frame, whilegenerating an FEC encoded frame.

Using the code-words specified in the standard as exemplary in theensuing description, the number of code-blocks Brs to be used totransmit a payload of M-bytes is determined by equation (1):

Brs=┌M/192┐.   (1)

A payload may thus be decomposed into a first (Brs-1) code-words of size(192,208) and a last code-word of size (Klast, Nlast) determined byequation (2):

$\begin{matrix}{\left( {{Klast},{Nlast}} \right) = \left\{ \begin{matrix}\left( {32,40} \right) & {{{if}\mspace{14mu} 0} < {M\mspace{14mu} {mod}\mspace{14mu} 192} \leq 32} \\\left( {64,74} \right) & {{{if}\mspace{14mu} 32} < {M\mspace{14mu} {mod}\mspace{14mu} 192} \leq 64} \\\left( {128,140} \right) & {{{if}\mspace{14mu} 64} < {M\mspace{14mu} {mod}\mspace{14mu} 192} \leq 128} \\\left( {192,208} \right) & {otherwise}\end{matrix} \right.} & (2)\end{matrix}$

RS-codes belong to the class of systematic codes i.e., codes where theresultant encoded code-word comprises of the original data suffixed bythe parity information. Recognizing this property, the elimination ofthe FEC-padding of the payload by transmitting a shortened lastcode-word is advocated, as described in the following.

The amount of FEC-pad i.e. MFECPad that would be required to be appendedto a payload prior to FEC encoding, such that the resultant FEC-encodedpacket is constituted of Brs code words as determined above, is given byequation (3), below. Consequently, the amount of information containedin the last code-word KlastAct is determined in equation (4).

MFECPad=192*(Brs−1)+Klast−M   (3)

KlastAct=Klast−MfecPad   (4)

A transmitter pads the information to be placed in the lastcode-word—KlastAct-bytes, with MFECPad-bytes of known values (which arealso known to the receiver). The transmitter encodes the resultantKlast-bytes to determine a code-word of Nlast-bytes.

However, in order to optimize usage of the medium, the transmittertransmits a shortened-last code word comprising of the KlastActinformation bytes suffixed with the (Nlast−Klast) parity bytesdetermined during the encoding process. Thus the last code-word has alength of KlastAct+(Nlast−Klast) bytes.

Correspondingly, a receiver incorporating the methods embodied hereininserts a pad of MFECPad-bytes equivalent to what was used as part ofencoding by the transmitter in between the KlastAct bytes of informationand the (Nlast−Klast) bytes of parity. The resultant Nlast bytescode-word is decoded using conventional RS-decoding methods.

The length of the shortened FEC-encoded frame L, is thus given byequation (5).

L=208*(Brs−1)+KlastAct+(Nlast−Klast)   (5)

FIG. 2 depicts an example application of FEC code-shortening. Frame 201represents a payload of M=264-bytes. The amount of FEC-padding MFECPadthat aligns the payload M to an integer number of code-words Brs, whileminimizing the overall parity overhead is determined using equations (1)through (3). Thus, as per the present example, Brs=2. The payload may besplit into a first block 211 of size 192-Bytes and a second block212—which comprises of 72-Bytes of data 212 a and 56-bytes of FEC-pad212 b. The split payload is FEC encoded, the first codeword 221(corresponding to block 211) being of type (192,208), while the secondcodeword 222 (corresponding to block 212) being of type(Klast,Nlast)=(128,140). Codeword 222 comprises of 72-Bytes ofinformation 222 a, 56-bytes of FEC-pad 222 b and 12-bytes of parity 222c. The shortened FEC encoded frame 230 may then be represented by afirst code word 231 (corresponding to codeword 221) and a shortened lastcodeword 232 comprising the information 232 a (corresponding toinformation 222 a) and the parity 232 c (corresponding to parity 222 c).

While described in the context of a MoCA system [1] using RS encoding,it would be apparent to one skilled in the art that similar methodswould apply to alternate coding schemes e.g., LDPC; and alternatecommunications systems.

The use of the redundant ACMT-pad which is appended to a payload priorto subcarrier modulation mapping such that the resultant subcarriermodulation mapped frame is comprised of an integer number of 256-toneACMT symbols, is reduced by means of adopting variable sized ACMTsymbols.

A transmitter on transforming a MAC packet to a PHY-packet prior totransmission reduces/eliminates the need for ACMT-pad by using ashortened ACMT symbol having a reduced number of tones, and consequentlya lower medium occupancy. The shortened ACMT symbol is selected from aset of sizes known a-priori to both transmitter and receiver. The samecyclic-prefix is used on both shortened as well as legacy 256-tone ACMTsymbols.

In another aspect, a transmitter exploits the fact that larger ACMTsymbols are more efficient i.e. have lower cyclic-prefix overheads thansmaller ones. Such a transmitter performs subcarrier modulation mappingwith the objective of using the largest available and applicable ACMTsymbols first, before attempting to use shorter symbols.

As an example, we shall consider the use of shortened symbols whoselength is a sub-multiple of the legacy 256-tone symbol. In the ensuingdescription, we shall assume a set of allowable ACMT symbols as given by{32, 64, 128, 256} tones. However, it would be apparent that the set ofallowable ACMT symbols may be extended to include other symbol sizes aswell.

Continuing with our illustration of a MAC-payload of M-bytes which wasencoded into a shortened FEC-encoded frame of length L, as per equation(5), a transmitter determines the number of ACMT symbols Nsym to betransmitted by applying equation (6).

Nsym=┌8*L/Nbas256┐  (6)

Where, Nbas256 refers to the number of bits that can be accommodated ina 256-tone ACMT symbol. The value Nbas256 is determined as part ofmodulation profiling as described in the standard.

Additionally, nodes can be further configured to determine the number ofbits per ACMT symbol for all available symbol sizes during modulationprofiling. As per the present example, nodes predetermine Nbas32,Nbas64, Nbas128 and Nbas256.

Thus, in the present example, a subcarrier modulation mapped frame wouldcomprise of a first (Nsym-1) 256-tone ACMT symbols and a last tailsymbol as determined by equations (7) and (8), below.

modL=(8*L) mod Nbas256   (7)

where, modL determines the number of bits to be modulated into the tailACMT symbol.

$\begin{matrix}{{Nbaslast} = \left\{ \begin{matrix}{{Nbas}\; 32} & {{if}\mspace{14mu} \begin{matrix}{0 < {{mod}\; L} \leq} \\{N\; {bas}\; 32}\end{matrix}} & {//{{tail}\mspace{14mu} A\; C\; M\; T\mspace{14mu} {symbol}\mspace{14mu} {is}\mspace{14mu} 32\text{-}{tone}}} \\{{Nbas}\; 64} & {{if}\mspace{11mu} \begin{matrix}{\; {32 < {{mod}\; L} \leq}} \\{N\; {bas}\; 64}\end{matrix}} & {//{{tail}\mspace{14mu} A\; C\; M\; T\mspace{14mu} {symbol}\mspace{14mu} {is}\mspace{14mu} 64\text{-}{tone}}} \\{{Nbas}\; 128} & {{if}\mspace{14mu} \begin{matrix}{64 < {{mod}\; L} \leq} \\{{Nbas}\; 128}\end{matrix}} & {//{{tail}\mspace{14mu} A\; C\; M\; T\mspace{14mu} {symbol}\mspace{14mu} {is}\mspace{14mu} 128\text{-}{tone}}} \\{{Nbas}\; 265} & {otherwise} & {//{{tail}\mspace{14mu} A\; C\; M\; T\mspace{14mu} {symbol}\mspace{14mu} {is}\mspace{14mu} 256\text{-}{tone}}}\end{matrix} \right.} & (8)\end{matrix}$

where, Nbaslast represents the number of bits per ACMT symbol of thelast/tail ACMT symbol.

The required ACMT-pad, MACMTpad to be added to the payload prior tosubcarrier modulation mapping is thus determined by equation (9).

MACMTpad=┌((Nsym−1)*Nbas256+Nbaslast)/8┐−L   (9)

While it would be apparent that the most efficient subcarrier modulationmapped frame that minimizes medium occupancy may not be comprised of aplurality of the longest ACMT symbol followed by a shortened tail ACMTsymbol i.e., a more efficient subcarrier modulation mapped frame couldhave been, for example constructed using a plurality of shorter ACMTsymbols, the above-mentioned mechanism minimizes the additionalsignaling required in the Asynchronous data/control reservationrequest-element and data-allocation-unit (DAU) that are required by theMoCA MAC protocol, as described in a subsequent embodiment. However, itis understood that all such variants of reducing the overhead of theACMT-pad by means of using variable-sized ACMT symbols are applicable asis apparent to one of ordinary skill in the art.

FIG. 3 depicts an example application of tail-ACMT symbol shortening.Carrying forward with the previous example of a 264-Byte frame, thelength of the shortened FEC-encoded frame L=292 was determined using(5). The number of symbols Nsym is determined as per equation (6) andthe size of the last symbol and the number of ACMT-pad bytes required isdetermined by equations (7) through (9). As, as example, consideringNbas256=1000; Nbas128=500; Nbas64=250 and Nbas32=125 and applying thedescribed methods, a 292-byte payload 301 would be appended with a21-byte ACMT-pad 311 to form a padded-frame 310. The padded frame 310 isthen split into an ACMT-modulated frame 320 constituted by twosymbols—321 and 322 of the maximum symbol-size and a shortenedtail-symbol of 128-tones 323.

FIG. 4 depicts the steps carried out by a transmitter in converting aMAC-packet to a PHY-packet for transmission over the channel. Packet 401depicts a MAC-frame of m-bytes which is handed to the PHY fortransmission. The PHY performs DES-padding by appending a DES-pad 406 ofup to 7-bytes so as to generate a DES-padded frame 405 of lengthM-bytes, where M is a multiple of 8. This satisfies the requirements ofthe standards based DES encryption which operates on multiples of8-bytes of data. The size of the DES-pad MDESpad is determined as perequation (10) and the DES-padded MAC frame has size M, as determined byequation (11).

MDESpad=8−(m mod 8)   (10)

M=m+MDESpad   (11)

The DES-padded frame 405 is encrypted using the DES encryption scheme toproduce an encrypted frame 410. It would be apparent that therequirement of DES padding to a multiple of 8-bytes is characteristic ofthe DES algorithm itself and is performed here in order to integrate thenodes into a MoCA-system. In a system that does use DES the steps ofgenerating 405 need not be used.

The PHY performs FEC-padding by appending MFECpad bytes of apre-determined FEC-pad 416 to the encrypted frame 410 to produce anFEC-padded frame 415. The FEC padded frame 415 is encoded intoindividual code-blocks, each code block constituted of a data-sectionand a parity section. As an example, we depict the encoding of 415 intotwo FEC code blocks 421 and 422, each of which is constituted of a datasection—421 a and 422 a, and a parity section—421 b and 422 b,respectively. The FEC encoded frame is collectively referred to as 420.

FEC code block 421 would have the largest code-word size (lowest parityoverhead); while FEC code block 422 may be one of the availablecode-words. Additionally, FEC code block 422 may be transmitted as ashortened code-word, having a data-section 423 a of size KlastAct bytes,as determined in equation (4), and a parity section 423 b (equivalent toparity-section 422 b of FEC code block 422) of (Nlast−Klast) bytes.Collectively, the shortened FEC encoded frame is referred to by literal424 in FIG. 4.

An ACMT-pad 426 of length MACMTpad, as determined in equation (9) issuffixed to the shortened FEC encoded frame 424 to produce and ACMTpadded frame 425. The resultant frame 425 is byte scrambled to producethe Byte-scrambled frame 430.

The byte-scrambled frame 430 is decomposed into an integer number (threeas per this example) ACMT symbols—435 a, 435 b and 435 c, collectivelycalled the subcarrier modulation mapped frame 435.

Symbols 435 a and 435 b would be the longest available and applicableACMT symbol, while symbol 435 c may be any one of the available ACMTsymbols. The length of symbol 435 c may be determined as per equation(8).

The symbols of the subcarrier modulation mapped frame 435 arebin-scrambled to produce a bin-scrambled frame 440. The PHY performsACMT modulation on 435 and inserts the appropriate preamble 446 togenerate an ACMT modulated frame 445. Frame 445 is further filtered andup-converted to the appropriate RF-carrier frequency to generate thefinal PHY packet 450, which is transmitted on the channel.

Nodes embodying the methods contained herein determine the bit-loadingprofile and the number of bits per ACMT symbol for all supported symbolsizes using the legacy Type-1 Probes as defined in the standard. For areduced-size ACMT symbol whose sub-carrier positions correspond to thesub-carrier positions of the Type-1 Probe's ACMT symbol, the per-toneSNRs (and consequently bit-loading) may be determined directly. For areduced-size ACMT symbol whose sub-carrier positions do not correspondto the subcarrier positions of the Type-1 Probe's ACMT symbol, theper-tone SNRs may be estimated by means of interpolation.

According to one aspect, the per-tone bit-loading pattern as determinedby the recipient of the Type-1 Probe frame may be communicated back tothe transmitter by means of extending the existing Type-1 Probe Report,as described in the standard by altering the LENGTH field as specifiedin the structure below to accommodate the bit-loading patterns for thenewly defined symbols—SHORT_BL_PATTERNn. Thus, the Type-1 Probe Reportmay be redefined to contain the following fields:

Type-1 Probe Report := {    PROBE_TYPE    NUM_ELEMENTS    REPORT_SOURCE   REPORT_RECEIVER    RELAY_FLAG    for ( i = 0; i < NUM_ELEMENTS; i++){      CHANNEL_SOURCE       CHANNEL_RECEIVER       PHY_PROFILE      PREAMBLE_TYPE       CHANNEL_USABLE       MAX_BINS      NUM_OF_SYMS       BITS_PER_ACMT_SYMBOL       CP_LENGTH      GCD_BITMASK       TPC_BACKOFF_MAJOR       TPC_BACKOFF_MINOR      for (j=0; j < 256; j++){          SC_MOD       }      SHORT_BL_PATTERNn    }    PAYLOAD_CRC }

A Type-1 Probe Report frame may contain a singularity or a plurality ofSHORT_BL_PATTERNn fields, depending on the number of supported ACMTsymbol sizes. In the instance of when a plurality of SHORT_BL_PATTERNnfields are included, the order of placement of SHORT_BL_PATTERNn fieldsin the Type-1 Probe Report should be predetermined in order tofacilitate correct interpretation of the frame at both transmitter andreceiver. As an example, we shall assume that the variousSHORT_BL_PATTERNn fields are arranged in descending order of symbol sizen.

A single SHORT_BL_PATTERNn field is defined as follows:

SHORT_BL_PATTERNn := {    for j=0; j<n; j++ {       SC_MODj    } }

Where SC_MODj refers to the bit-loading pattern applicable on tone j fora n-point ACMT symbol.

It is understood that new frame-type can be defined to carry bit-loadingprofiles of specific symbol sizes.

The recipient of a Type-1 Probe frame determines the bit-loading patternfor a 256-tone ACMT symbol and communicates this to the transmitter viathe Type-1 Probe Report Frame, as defined in the context of legacynodes. Nodes implemented as described here can further infer thebit-loading pattern of the available symbol-sizes by applying a commonset of rules on the legacy (256-tone) bit-loading pattern. The fact thattransmitter and receiver use the same rules would guarantee consistencybetween their respectively inferred modulation profiles for a givensymbol-size.

The bit-loading of tone k of a j-tone ACMT symbol may be determined asthe minimum of the bit-loading on the adjacent tones of the 256-toneACMT symbol. As an example, the bit loading of tone 551 of a 128-toneACMT symbol 550, may be determined as the minimum of the bit-loading ofthe adjacent tones—511 and 512 of the 256-tone ACMT symbol 510. As aconsequence of this method, nodes implemented as described here requireno additional signaling to effect the exchange of themodulation-profiles for different ACMT-symbol-sizes in a MoCA network.

It is understood that there can be several variations to the common setof rules practiced by the nodes, for example, the derived bit-loading ofa particular tone may be defined to be the mean of bit-loading acrossseveral tones of the 256-tone symbol; or in another realization, havinga back-off from the value determined above.

As discussed previously, the MoCA MAC protocol is built around TDMAwhere a node with data to transmit, first transmits a RR to the NC,which computes a schedule and broadcasts a MAP-frame defining theschedule of transmissions (in terms of AUs) over the next MAP-cycle.Nodes in the network then schedule their transmissions and reception forthe next MAP-cycle based on the AUs contained in the MAP-frame.

In order to correctly demodulate a PHY packet transmitted in accordancewith the method of FIG. 4, the receiver needs to be aware of the numberof ACMT-symbols, their respective symbol-sizes and modulation capacity(Nbas); the cyclic-prefix; the number and size of the FEC code-blocksused; and the amount of DES-pad applied. While MoCA systems of prior-artrequired knowledge of only the PHY-profile in use and the number of ACMTsymbols to correctly setup the receiver for reception, nodes embodyingthe methods contained herein, further need knowledge of the number ofACMT-pad bytes—MACMTpad.

The asynchronous-data/control request element that defines the bandwidthrequirements for a data/control transmission is modified to additionallycontain the MACMT_PAD field which defines the number of ACMT-pad bytesused in the transmission, as depicted in the structure below.

Asynchronous Data/Control Reservation Request Element :=    {      FRAME_SUBTYPE       FRAME_TYPE       DESTINATION       PHY_PROFILE      REQUEST_ID       PARAMETERS       PRIORITY       DURATION      MACMT_PAD    }

The value of MACMT_PAD can be accommodated in the unused bits (eg: thereserved PARAMETERS field) of the asynchronous data/control reservationrequest element as defined in [1].

The DAU which is used to allocate bandwidth to a node that requested forit using a corresponding asynchronous data/control reservation requestelement, as defined in [1] may similarly be modified to additionallycontain the MACMT_PAD field as depicted in the structure below:

Data Allocation Unit :=    {          FRAME_SUB_TYPE          FRAME_TYPE         SRC          DESTINATION          PHY_PROFILE         REQUEST_ID          IFG_TYPE          OFFSET          MACMT_PAD   }

The MACMT_PAD field can be accommodated in the unused/reserved bits (eg:the excess bits of the SRC and DESTINATION fields) of the DAU frame asdefined in [1].

FIG. 6 is a flowchart describing the method to be adopted by atransmitter to determine the DURATION and MACMT_PAD parameters of theasynchronous data/control reservation request as defined above, prior totransmission of a packet as per the steps of FIG. 4.

The flowchart is invoked in step 600, when there is a m-byte frame to betransmitted. In step 610, the size of the DES-pad and consequently theDES-padded frame is determined in using equations (10) through (11). Instep 620, the size of the FEC-pad to be applied and the number and sizesof the various FEC code words is determined by means of equations (1)through (4). Further the size of the shortened FEC-encoded frame L isdetermined using equation (5).

In step 630, the amount of ACMT-pad to be applied in order to constructan ACMT-modulated frame is determined, applying equations (6) through(9).

In step 640, the duration of the packet transmission is determined basedon the number and sizes of the ACMT-symbols as determined in step 630and the cyclic-prefix and preamble-type in use, using the methodspecified in [1].

The flowchart terminates in step 650.

FIG. 7 is a flowchart describing the method to be adopted by a receiverto determine the necessary parameters to correctly receive a PHY packetdescribed by the DAU, as defined above.

The flowchart is invoked in step 700, on receiving a MAP-frame with aDAU indicating an impending reception to the receiver. In step 710, theduration of the transmission Nsamp is computed based on the differencein the OFFSET fields of the DAU of interest and the subsequent AUcontained in the MAP-frame. Using the cyclic prefix CPlen andpreamble-length PreambleLen (based on the PHY-profile indexed by the3-tuple—{SRC, DESTINATION, PHY_PROFILE} contained in the DAU), thenumber of ACMT symbols Nsym and the length of the tail ACMT symbol Tsymis determined in 720, as per equations (12) and (13).

Nsym=┌(Nsamp−PreambleLen)/(256+CPlen)┐  (12)

Where, PreambleLen is the length of the preamble in number of samplesappropriately adjusted in accordance with the conventions of standard.

Tsym=Nsamp−(Nsym−1)*(256+CPlen)−CPlen   (13)

Nbaslast is selected form the number of bits per ACMT symbol for thevarious symbol-sizes using Tsym. In step 730, the number of bytes in theshortened FEC-encoded frame L as transmitted in 424 of FIG. 4 isdetermined using equation (14):

L=┌((Nsym−1)*Nbas256+Nbaslast)/8┐−MACMTpad   (14)

In step 740, the number of FEC code-words Brs and the size of the lastFEC code-word FEClast of the received packet are determined as perequations (15) and (16).

Brs=┌L/208┐  (15)

FEClast=L mod 208   (16)

As per the method of the transmitter previously described in FIG. 2, thefirst (Brs−1) code words are of type (192,208), while the parameters ofthe last FEC code-word are determined as in equations (17) and (18):

$\begin{matrix}{\left( {{Klast},{Nlast}} \right) = \left\{ \begin{matrix}\left( {32,40} \right) & {{{if}\mspace{14mu} 0} < {FEClast} \leq 32} \\\left( {64,74} \right) & {{{if}\mspace{14mu} 40} < {FEClast} \leq 64} \\\left( {128,140} \right) & {{{if}\mspace{14mu} 74} < {FEClast} \leq 128} \\\left( {192,208} \right) & {otherwise}\end{matrix} \right.} & (17) \\{{KlastAct} = {{FEClast} - \left( {{Klast} - {Nlast}} \right)}} & (18)\end{matrix}$

The flowchart of FIG. 7 terminates in step 750, with the receiver havingdetermined the number and sizes of the ACMT symbols and the number andsizes of the FEC-code-words from the DAU. The receiver can then be setupfor correct reception of the packet.

A receiver can determine the size of the last ACMT symbol to be receivedby it, prior to actual reception, based on decoding the AUs of theMAP-frame, as described in a previous embodiment. The receiver uses thisinformation along with the received (legacy) 256-tone channel estimationsequence to determine an appropriate set of channel estimatescorresponding to the tone positions of the shortened ACMT symbol,facilitating its subsequent demodulation. The channel estimates to beapplied to the shortened ACMT symbol may be based on interpolationacross tones of the channel estimates of the 256-tone channel estimationsequence. Thus, as per the methods embodied herein, the transmission ofadditional channel estimation sequences for reduced size ACMT symbols(in addition to legacy channel estimation sequences) are not required inorder to practice the method of transmitting a shortened ACMT symbol. Itis understood however that additional channel estimation sequences forthe reduced-size ACMT symbol can be used.

As an example, consider the tones of 256-tone ACMT symbol 510—forexample tone-position 511 and tone-position 512 as representing thechannel estimates at two adjacent tones, estimated from a 256-tonechannel estimation sequence. The channel estimate of a correspondingtone-position 551 of a reduced size 128-tone ACMT symbol may bedetermined by interpolating across the channel estimates 511 and 512. Itwould be apparent to one skilled in the art that the channel estimatesof other adjacent tones from 256-tone ACMT-symbol 510 may also be usedto determine the channel estimate at tone-position 551.

The performance enhancements realizable by nodes incorporating theteachings embodied herein may be greatly enhanced by an NC not using the‘2600-slot limit’ between two transmissions in the network as defined inthe standard.

It would be apparent to one skilled in the MoCA standard [1], that themethods incorporated herein may be practiced by a subset of nodes in aMoCA network to achieve reductions in medium occupancy duringtransmissions between them. As such, it would be apparent that a nodeincorporating the present invention may communicate with legacy nodes byreverting to means of communications specified by the standard. It wouldbe apparent that the operation of these legacy-nodes would not behampered by the nodes practicing the present invention. Thus, it isenvisioned that nodes incorporating the present invention would beinter-operable and could coexist in a network with legacy nodes.

While certain embodiments of the invention have been described above, itwill be understood that the embodiments are by way of example only.Accordingly, the invention should not be limited based on the describedembodiments.

1. A method for generating a Forward Error Correction (FEC) encodedframe for use in communicating data between nodes over a network, themethod comprising: identifying a set of FEC code-words having differentsizes, the set of FEC code-words including a largest size FEC code-word;encoding as much of a payload as possible into one or more FECcode-words of the largest size FEC code-word; and encoding a remainderportion of the payload into a last FEC code-word, the last FEC code-wordbeing the smallest possible FEC code-word of the set of FEC code-words;wherein the combination of the one or more largest size FEC code-wordsand the last FEC code-word forms a shortened FEC encoded frame.
 2. Themethod of claim 1 further comprises adding an FEC pad to the payload,wherein the size of the FEC pad is selected so that the remainderportion of the payload and the FEC pad fill the last FEC code-word. 3.The method of claim 2 further comprising removing the FEC pad togenerate a shortened last FEC code-word and using the shortened last FECcode-word to form the shortened FEC encoded frame.
 4. The method ofclaim 1 wherein the set of FEC code-words comprises FEC code-words ofsizes (32, 40), (36, 44), (64, 74), (128, 140), and (192, 208) bytes. 5.The method of claim 1 further comprising: reducing the adaptiveconstellation multi-tone (ACMT) pad overhead required to ACMT modulate adata frame to form an ACMT-modulated frame, the reduction of saidACMT-pad resulting in a shortened ACMT modulated frame, said methodcomprising the steps of: determining a number of ACMT symbols thatreduces the medium-occupancy of the associated ACMT modulated frame; anddetermining a reduced ACMT pad to be appended to the data frame, theresultant ACMT padded data frame, when modulated, resulting in thenumber of ACMT symbols, the resultant ACMT symbols being of sizes thatreduce medium occupancy, the aggregate of the ACMT modulated symbolsbeing referred to as a shortened ACMT modulated frame.
 6. The method ofclaim 1 further comprising generating a shortened adaptive constellationmulti-tone (ACMT) modulated frame from the shortened FEC encoded frame,wherein generating the shortened ACMT modulated frame comprises:establishing a set of ACMT modulation symbols having different sizes,the set of ACMT modulation symbols including a largest size ACMTmodulation symbol; encoding as much of the shortened FEC encoded frameas possible into one or more ACMT modulation symbols of the largest sizeACMT modulation symbol; and encoding a remainder portion of theshortened FEC encoded frame into a tail ACMT modulation symbol, the tailACMT modulation symbol being the smallest possible ACMT modulationsymbol of the set of ACMT modulation symbols; wherein the combination ofthe one or more largest size ACMT modulation symbols and the tail ACMTmodulation symbol form the shortened ACMT modulated frame.
 7. The methodof claim 6 further comprising adding an ACMT pad to the shortened FECencoded frame, wherein the size of the ACMT pad is selected so that theremainder portion of the shortened FEC encoded frame and the ACMT padfill the tail ACMT modulation symbol.
 8. The method of claim 6 whereinthe set of ACMT modulation symbols comprises ACMT modulation symbols of32, 64, 128, and 256 tones.
 9. A method for generating an adaptiveconstellation multi-tone (ACMT) modulated frame for use in communicatingdata between nodes over a network, the method comprising: establishing aset of ACMT modulation symbols having different sizes, the set of ACMTmodulation symbols including a largest size ACMT modulation symbol;encoding as much of a payload as possible into one or more ACMTmodulation symbols of the largest size ACMT modulation symbol; andencoding a remainder portion of the payload into a tail ACMT modulationsymbol, the tail ACMT modulation symbol being the smallest possible ACMTmodulation symbol of the set of ACMT modulation symbols; wherein thecombination of the one or more largest size ACMT modulation symbols andthe tail ACMT modulation symbol form a shortened ACMT modulated frame.10. The method of claim 9 further comprising adding an ACMT pad to thepayload, wherein the size of the ACMT pad is selected so that theremainder portion of the payload and the ACMT pad fill the tail ACMTmodulation symbol.
 11. The method of claim 9 wherein the set of ACMTmodulation symbols comprises ACMT modulation symbols of 32, 64, 128, and256 tones.
 12. The method of claim 9 further comprising determining anumber of ACMT modulation symbols that reduces the medium-occupancy ofthe associated ACMT modulated frame.
 13. A method for decoding a ForwardError Correction (FEC) encoded frame that is used in communicating databetween nodes over a network, the method comprising: inserting an FECpad of a predetermined content and size in between the data and parityof a shortened last FEC code-word of a shortened FEC encoded frame,resulting in an FEC encoded frame; decoding the FEC encoded frame todetermine an FEC padded data frame; and discarding the FEC pad torecover an underlying data frame.
 14. A method for determining amodulation profile for use in communicating data between nodes over anetwork, the method comprising: transmitting a legacy modulationprofiling sequence from a transmitter to a receiver; using the receivedmodulation profiling sequence to determine the signal to noise ratio oflegacy tone positions; and determining the signal-to-noise ratio at thetone positions of a shortened adaptive constellation multi-tone (ACMT)modulation symbol by interpolation of the signal-to-noise ratios of thelegacy tone-positions.
 15. The method of claim 14 further comprisingcommunicating the determined modulation profile of the reduced-size ACMTsymbol from the transmitter to the transmitter.
 16. A method determininga modulation profile for use in communicating data between nodes over anetwork, the method comprising: determining a modulation profile for ashortened adaptive constellation multi-tone (ACMT) modulation symbolfrom a modulation profile of a legacy ACMT modulation symbol, thedetermination including applying a common and pre-determined set ofrules to the modulation profile of the legacy ACMT symbol to determinethe modulation profile of the shortened ACMT modulation symbol.
 17. Themethod of claim 16 wherein the bit-loading capacity of a tone of ashortened ACMT modulation symbol is determined as the minimum of thebit-loading capacities of adjacent tones of the legacy ACMT modulationsymbol.
 18. The method of claim 16 further comprising communicating apreamble type in use by a transmitter, the modulation profile used bythe transmitter in transmitting a shortened ACMT modulated frame, thecyclic-prefix in use by the transmitter for every ACMT symbol, theduration of the transmission, and the ACMT-pad used by the transmitterin its transmission.
 19. The method of claim 16 further comprisingdetermining channel estimates at tone positions of the shortened ACMTsymbol based on channel estimates at tone positions of legacy ACMTmodulation symbols as determined from a legacy preamble by determiningthe size of the shortened ACMT modulation symbol prior to receiving itand interpolating the channel estimates at the tone positions of thelegacy ACMT modulation symbol to determine the channel estimates at thetone positions of the shortened ACMT modulation symbol.